concrete cover around reinforcement can be deteriorated by the surrounding
environment, because of various origins:
physical causes: freezing, etc;
mechanical causes: concrete can crack under an excessive loading,
chemical causes, for example, due to bodies (gas or ions) contained
in the environment.
As a rule, reinforced concrete structures are in contact with atmosphere,
with water (river, sea, etc.) or with ground. These environments
are more or less polluted and contain some bodies (gas or liquid)
which can enter concrete and modify its characteristics, particularly
the chemical composition of its pore solution.
The most frequent aggressive agents are pure water, chlorides dissolved
in water and carbon dioxide (CO2) in atmospheres.
can wash out (leaching)
concrete by dissolving some cement components and increase concrete
Chloride are highly soluble in water. So, these ions are dissolved in water,
and penetrate with it in concrete (chlorides
penetration), either by the wetting of a non saturated concrete
(convection), or by diffusion
because the chloride content is higher in the environment than in
the original concrete (concentration gradient). The main part of
chlorides coming from outside remain as dissolved ions, in the concrete
pore solution. But they can also react with some components of the
material (chemical reaction or adsorption).
The carbon dioxide
(CO2) is a gas in the atmosphere. It can be dissolved by the concrete
pore solution, and react with some calcium compounds to form carbonates
So, the pH is about 9 for the pore solution of a concrete deteriorated
The penetration of carbon dioxide in concrete is a diffusion
process It is fast when the concrete is rather dry. But the carbonation
reaction does not occur if no pore solution is present in concrete.
This is the reason why, the most favourable conditions to the penetration
of carbon dioxide correspond to a relative humidity of about 65%.
Practical consequences : To slow
down the penetration of the aggressive agents, it is necessary to
formulate and to make concrete, so that its porosity is low and
the diffusion coefficients of these agents are low.